The present invention relates to inflatable packers, and particularly to an inflatable packer with a bladder that reliably and controllably expands downhole.
An inflatable packer is used in a downhole wellbore to seal the inside of the wellbore or a downhole tubular. The packer includes an inner mandrel and an outer expandable body which typically is constructed of three layers:
(1) a bladder or inner elastomer layer;
(2) a reinforcement layer or layers with a reinforcing material, such as slats or cable; and
(3) a cover or outer elastomer layer.
A portion of the lower-friction elastomer cover is commonly removed so the higher-friction reinforcement layer contacts the borehole wall, providing a more secure fit between the inflated packer and the borehole. Fluid pressure is applied in the space between the OD of the mandrel and the ID of the bladder layer surrounding the mandrel to expand the packer inside the borehole. As the bladder expands, it causes the surrounding reinforcement layer and cover layer to expand against the borehole.
Due to variations in the cross-section of the borehole, irregularities in the construction (geometry) or material homogenity of either the bladder, the reinforcement layer or the cover, or due to the different expansion where the end of the cover exposes the reinforcement layer, the bladder may expand non-uniformly, causing the bladder to fold at one or more locations. The non-uniform expansion increases the overall length of the bladder, and that excess length may accumulate in the area referred to as a xe2x80x9cZ-fold.xe2x80x9d Some areas within and adjacent to the Z-fold may be highly stretched due to expansion, and other areas adjacent to the Z-fold may be compressed due to the excess length. The Z-fold may worsen with increased pressure and expansion, overstressing the bladder. If stresses exceed the elastic limit of the elastomer or bladder, the packer will fail. Packer failure may result in hundreds of thousands of dollars expended to replace the failed packer, and to repair the damage to other downhole tools or the formation.
Elastomers used in bladders fail predictably when stretched beyond their elastic limit, which limit may be expressed as a percentage. As a precaution, the manufacturer will usually specify a packer whose maximum stress in a given application will be substantially lower than the elastic limit. For example, if the elastomer used in a packer has an elastic limit of 600% elongation, the manufacturer may recommend using it in applications in which the maximum stretch will not exceed 400%, with further downward adjustments for high-temperature or harsh environments. Despite such precautions, packers often fail as a result of Z-folding, and replacement can be time consuming and costly.
Numerous patents have addressed techniques to control bladder expansion to minimize the risk of failure. Patents of interest include U.S. Pat. Nos. 6,315,053; 6,223,820; 6,158,506; 5,813,459; 5,613,555; 5,605,195; 5,564,504; 4,967,846; 4,886,117; and 4,768,590.
The disadvantages of the prior art are overcome by the present invention, and an improved inflatable packer is disclosed below with a packer element or bladder offering more reliable and controlled expansion.
This invention discloses an improved technique for controlling expansion of a packer to minimize folding of the bladder, commonly referred to as Z-folding, thereby improving the reliability of the expansion process and reducing the likelihood of failure.
In one embodiment, a sleeve-shaped bladder surrounds a packer tube, and a radially outward reinforcement layer extends axially between upper and lower packer subs. A cover may be provided only over a limited portion of the reinforcement layer so that when the packer expands, the exposed portion of the reinforcement layer engages the ID of the wellbore.
To minimize the likelihood of Z-folds, the bladder is placed in tension by stretching it axially between the upper and lower packer subs prior to inflation. This may be done either before or after the packer is placed in the wellbore. The pre-stretched configuration allows the bladder to inflate uniformly, because the tension in the bladder essentially pulls out the Z-fold as it forms. Because the bladder is in tension, the region within and adjacent to a Z-fold that would ordinarily be compressed will instead merely experience a reduction in tension. Pre-stretching essentially removes the excess length that would otherwise accumulate in a Z-fold.
It is an object of the present invention to provide an inflatable packer with an elastomeric bladder stretched between the upper and lower packer subs to prevent Z-folding. Prior to inflation, the bladder is axially stretched an amount equal to the elongation it might otherwise experience as a result of folding.
A related object of the invention is to provide an improved method of inflating a packer downhole in a wellbore by pre-stretching the bladder between the upper packer sub and the lower packer sub to minimize the occurrence of Z-folds.
It is a feature of the invention that both the upper and lower packer sub of the assembled packer may be substantially fixed with respect to the packer tube, so that the bladder is axially stretched during manufacture of the packer.
Another feature of this invention is that at least one of the upper packer sub and the lower packer sub may be axially movable relative to the packer tube while the packer is downhole to pre-stretch the bladder downhole prior to inflation. Hydraulic or pneumatic pressure may be used to move the movable sub axially relative to the stationary sub to pre-stretch the bladder prior to inflation of the packer. In another embodiment, a mechanical biasing member such as a spring may be used to move the movable sub axially relative to the stationary sub to pre-stretch the bladder prior to inflation of the packer.
A further feature of this invention is that the packer may include a reinforcement layer surrounding at least a portion of the bladder for engaging the inside of a wellbore. The bladder may be pre-stretched prior to fitting the bladder between the OD of the packer tube and the ID of the reinforcement layer.
A further feature of the invention is that the elastomeric bladder may preferably be pre-stretched a fixed amount of at least 10%, commonly at least 25%, or preferably at least 40% of its unstretched length, which may provide high reliability against folding, while remaining substantially below the elastic limit of the bladder. Pre-stretching a selected fixed amount allows a single packer to be used in one of a variety of different applications and expansion ratios.
A significant advantage of the present invention is that the reliability of a packer may be substantially improved.
A related advantage is a reliable packer having a pre-stretched bladder may be constructed with a minimal increase in cost as compared with prior art packers. The cost to pre-stretch the bladder is small relative to the overall cost of the packer. Yet another advantage of pre-stretching a bladder by a fixed amount is the bladder may be easier and less expensive to manufacture than a packer whose bladder is custom-stretched for a specific application. A bladder pre-stretched by a fixed amount may protect against Z-folds in a variety of packer applications.
Still another advantage of a packer having a moveable packer sub is the user may custom stretch a packer to adapt it to a chosen application.
These and further objects, feature, and advantages of the present invention will become apparent from the following detailed description, when reference is made to the figures in the accompanying drawings.